The development of hot melt construction adhesives for disposable consumer articles has paralleled the increasing complexity of the articles themselves. The increasing complexity of disposable articles requires hot melt construction adhesives to bond to a wider range of substrates such as various nonwoven materials and low surface energy polyolefins such as untreated polypropylene. The use of thinner polyolefin back sheets in the manufacture of disposable articles requires the use of lower viscosity hot melts in order to prevent burn-through and distortion when the adhesive is applied. Highly creep resistant bonds are required to hold elastic attachments in place, for example, elastic attachments made from natural rubber, polyurethane, and various types of foam. In addition, restrictions on low odor and light color of the hot melts are needed to meet consumer expectations.
Hot melt construction adhesives for disposable articles are known, including ethylene-vinyl acetate (EVA) copolymer-based hot melts, amorphous polypropylene-alpha-olefin (APAO)-based hot melts, styrene-butadiene-styrene (SBS) A-B-A-B-A multi-block copolymer-based hot melts, styrene-butadiene-styrene (SB).sub.n radial copolymer-based hot melts, and styrene-isoprene-styrene (SIS) A-B-A block copolymer-based hot melts.
EVA copolymer-based hot melts such as those disclosed in U.S. Pat. No. 4,299,475 suffer from poor adhesion to polyolefins and require large add-ons to obtain sufficient bond strengths.
APAO-based hot melts such as those disclosed in U.S. Pat. No. 4,460,728 lack specific adhesion to polyolefins and exhibit poor creep resistance and poor machinability.
SBS multi-block copolymer-based hot melts such as those disclosed in U.S. Pat. No. 4,526,577 possess poor specific adhesion to foam elastic attachments used in leg gathers and waist bands on diapers and adult incontinent articles and inferior creep resistance (particularly noticeable at low temperatures which articles are commonly exposed to during shipment and storage). Furthermore, SBS multi-block copolymer-based hot melts exhibit thermal stability problems; i.e., when they are subjected to heating during the article manufacturing process, these hot melts have a tendency to gel and plug the application equipment.
Hot melt construction adhesives based on styrene-butadiene-styrene (SB).sub.n radial copolymers such as those disclosed in U.S. Pat. Nos. 4,944,933, 5,024,667, 5,037,411 and 5,057,571 suffer from poor machining and adhesion limitations which result in inferior bonds to polyolefin substrates. In addition, these formulations are highly over-tackified to provide necessary creep resistance, which in turn renders the adhesives very stiff at low temperatures and thus yields inferior bond strengths at those low temperatures. Radial polymers, while inherently stronger than multi-block copolymers, do not exhibit desirable adhesion characteristics, mainly due to the positioning of the styrene domains. Multi-block copolymers have more mid-block ends extending away from the styrene domains. Since the mid block dictates the adhesive properties of these hot melts, polymers possessing more mid-block ends tend to exhibit better adhesion characteristics.
Hot melt construction adhesives based on styrene-isoprene-styrene A-B-A block copolymers such as those disclosed in U.S. Pat. No. 5,149,741 possessing high amounts of styrene, without styrene endblock modifying resins, suffer from a loss of creep and heat resistance upon long exposure to high temperatures. SIS copolymers undergo chain scission reactions which form large amounts of styrene-isoprene (SI) diblocks. This causes a loss in mechanical properties which manifests itself in dramatic decreases in viscosity, lower heat resistance, lower shear adhesion failure temperature, and lower creep resistance as indicated by a shorter time to failure during static shear at both room temperature and elevated temperature. Furthermore, these compositions are highly over-tackified, thus deleteriously impacting their low temperature adhesion.
Non-pressure sensitive hot melt adhesives based on A-B-A styrene-butadiene-styrene copolymers, A-B-A-B-A styrene-butadiene-styrene copolymers, and their hydrogenated counterparts (styrene content 15 to 60%) such as those disclosed in U.S. Pat. No. 5,275,589 are specifically designed to bond to the outer nonwoven layer of three ply diaper construction and are high viscosity oil-less formulations.
Sprayable hot melt adhesives based on blends of styrenic block copolymers (12 to 45% styrene by weight), tackifying resins, and ethylene copolymers such as ethylene-methacrylate copolymers such as those disclosed in U.S. Pat. No. 5,401,792 are well known in the industry to be highly unstable systems, which can cause equipment problems due to gelation upon overexposure to high temperature.
Pressure sensitive, low tack hot melt adhesive compositions containing styrene-butadiene radial block copolymers, styrene-isoprene radial block copolymers and/or styrene-isoprene linear block copolymers, and plasticizing oils, such as those disclosed in U.S. Pat. No. 5,523,343, exhibit a good balance of properties normally required. However, these hot melt adhesive compositions possess very low tack values (i.e., peel adhesion and quick stick) and are very high in viscosity. Accordingly, they are not viable adhesives for disposable article construction applications which require high tack valves and low viscosities at 140.degree. C., i.e., &lt;25,000 mPa.multidot.s.